Hard-Trim a VSE project movie clips for backup/storage
Automating the process would require some work, but https://superuser.com/questions/377343/cut-part-from-video-file-from-start-position-to-end-position-with-ffmpeg has some answers on how to make excerpts from larger videos.

TLDR:

ffmpeg -i in.mp4 -ss [start] -t [duration] -c copy out.mp4


Unfortunately, you'll have to create replacement strips that refer to the excerpt clip and get things frame-accurate again.

While I can imagine writing some python that creates these replacement strips, I am not confident that they will be frame accurate.